The present disclosure and various embodiments described herein relate in general to obtaining a position fix of a mobile device, and more specifically to methods and apparatuses for obtaining a position fix of a mobile device using hybrid positioning functionality.
Mobile devices that have the ability to obtain its terrestrial position often do so using Global Positioning System (GPS) receivers and technology. The procedure for obtaining this position is sometimes referred to as obtaining a “position fix.” In the vast majority of cases, mobile devices with GPS receivers need to rely on only positioning satellites from a global navigation satellite system (GNSS) to obtain a position fix, such as the American satellite constellation GPS or the Russian system GLONASS (Global Orbiting Navigation Satellite System). Other global satellite systems are in production, such as the European system GALILEO and Chinese system COMPASS.
To calculate a position fix using only GNSS measurements, wireless signals concurrently transmitted from satellites at known locations are used in a process of calculating position of a GPS receiver. The GPS receiver receives the signals from satellites within its view of the sky. The GPS receiver measures the distance between itself and at least three GPS satellites by measuring time delay between transmission and reception of each GPS radio signal. Having the positions of three GPS satellites and distances between the GPS receiver calculates its position using a range-based calculation called trilateration.
For trilateration calculations it is necessary to know the time of transmission and the time of reception of the GPS signal. This allows calculating the distance using the known speed of the signal, which is the speed of light. However, due to even slight inaccuracies in the time keeping of mobile devices, the distances are not exact. Thus, a typical distance calculation includes an uncertainty, sometimes called a time bias or clock bias. The approximate distance calculation plus the clock bias is often called a pseudorange, and it is these pseudoranges that are actually used in trilateration techniques for obtaining a position fix.
To adjust for clock bias, often a fourth satellite in view can be used to solve for this fourth variable, or other times the altitude of the mobile device acts as the fourth constraint.
However, it is not always the case that a mobile device has three or four satellites in view. Mountains, urban areas, and even dense forests may prevent accurate reception from satellites to GPS receivers. Assistance data from location servers on the ground have been created over the years to help improve the accuracy of a position fix and to help compensate for limitations like these. Terrestrial base stations have been installed to help transmit signals to mobile devices that otherwise are not in view of a sufficient number of satellites. These base stations, location servers and other assistance apparatuses are connected via a wireless network to create a system for providing assistance data known as Assistance GPS or A-GPS.
In cases where there are less than three GNSS satellites in view, the A-GPS network has been used to help obtain a position fix by providing base station locations to effectively substitute for the missing satellites. Positioning processes that use multiple positioning technologies, like one or more GNSS satellites plus one or more base stations, are sometimes referred to as hybrid positioning.
There are various A-GPS networks used in telecommunications, built around various cellular communication system protocols, each with different message formats. For example, Radio Resource Location services (LCS) Protocol (RRLP) is used for the Global System for Mobile Communications (GSM) and the Universal Mobile Telecommunications System (UMTS), IS-801 is used for Code Division Multiple Access (CDMA), and Radio Resource Control (RRC) is used for Wideband CDMA (WCDMA) and the Open Mobile Alliance Secure User Plane Location (OMA SUPL).
However, given the global nature of wireless communications and a diversified array of telecommunications carriers that service the millions of mobile devices now, there has grown a need to create more uniform and standardized wireless protocols. The 3rd Generation Partnership Project (3GPP) is such an organization that seeks to create a universal standard so that wireless carriers, engineers and researchers can more efficiently improve wireless communications by developing on a common platform. It is argued that the field is converging toward a mobile communications standard called Long-Term Evolution (LTE), which places an emphasis in optimizing data storage and transmission. The LTE Positioning Protocol (LPP) is a message format standard developed for LTE and that defines the message format between a mobile device and the location servers that have been commonly used in A-GPS functionality. The expectations of some in the field are that LPP will be followed uniformly across most or all carriers, creating a need to optimize location servers to efficiently utilize these LPP message formats.
Two areas of message formats in LPP include OTDOA (Observed Time Difference of Arrival) measurements, and FTA (Fine Time Assistance) uplink information. OTDOA measurements pertain to downlink information to mobile devices, or user equipment (UE), from terrestrial base stations, or eNodeBs, as opposed to GNSS satellites and the like. The principle behind OTDOA positioning is similar to GPS. The location determination is typically distributed between the UE and the network. Unlike GPS positioning, the UE does not acquire an accurate reference time, but the position estimate is based on the received time difference of at least two base station cells.
FTA information includes information transmitted from a mobile device to a location server that is in addition to traditional timing and ephemeris data received from GNSS measurements. The information in FTA can be helpful to a location server in calculating more accurate position fixes. FTA information can be retrieved in various A-GPS networks, but their format and content can vary depending on the standard used. LPP in particular has a defined format that differs from all other A-GPS protocols currently devised.
While LPP exhibits many advantages for carriers, and may promise to be the wireless communications protocol standard of the future, the message format protocol also contains limitations as originally intended.